1. Field of the Invention
The present invention relates to a semiconductor fabrication method for growing a compound semiconductor on a semiconductor substrate (i.e., on a growth substrate) by a molecular beam epitaxy technique, and a semiconductor fabrication apparatus to be used for performing such a semiconductor fabrication method. In particular, the present invention relates to a semiconductor fabrication method capable of effectively simplifying the fabrication steps, particularly when a material having a high vapor pressure is used, and a semiconductor fabrication apparatus to be used for performing such a semiconductor fabrication method.
2. Description of the Related Art
In order to grow a high-quality compound semiconductor crystal to be used for such devices as semiconductor light-emitting devices or high-speed electron devices, a molecular beam epitaxy (MBE) technique, a metal-organic chemical vapor deposition (MOCVD) technique, a liquid phase epitaxy (LPE) technique, or the like, are typically employed. Among them, the MBE technique has many advantages such as its high controllability of growth of an ultra-thin film, ability to grow a fine crystal at a relatively low temperature, ability to effectively use growth materials, and the availability of in-situ observation of a growth state.
It has been attempted to grow various types of semiconductor materials using the MBE technique, taking advantage of the favorable characteristics of the MBE technique as described above. In fact, the MBE technique can handle many kinds of semiconductor materials, including a material having an extremely high vapor pressure such as phosphorus, selenium, and sulfur. As a result, excellent results have been obtained in fabricating such devices as semiconductor lasers or the like.
In the case where a material having a high vapor pressure is grown by the MBE technique, it becomes important to maintain a favorable degree of vacuum in a growth chamber during the crystal growth process. An example of the conventional technique which has been intended to realize the above will described below, taking the case where a red-color light emitting semiconductor laser device (hereinafter, also simply referred to as the "red semiconductor laser") is grown as an example.
FIG. 7 shows a schematic configuration of one conventional example of an MBE apparatus, i.e., a semiconductor fabrication apparatus to be used for obtaining a red semiconductor laser using the MBE technique. The use of this type of the MBE apparatus for obtaining a practical red semiconductor laser is described, for example, in T. Hayakawa et al.: Journal of Crystal Growth, 95, (1989), pp.343-347 (hereinafter referred to as "Document 1"); and K. Takahashi et al.: Journal of Crystal Growth, 150, (1995), pp.1333-1337 (hereinafter referred to as "Document 2").
Document 1 refers, in its Conclusion section, to the fabrication of an AlGaInP type red semiconductor laser using solid materials through the MBE technique as one example. Document 1 further reports that the use of the MBE technique allows a thickness of a grown layer and various parameters of a doping process to be well-controlled, and further enables obtaining a uniform crystal. In particular, it is reported that a p-type cladding layer can be doped at a high concentration of 10.sup.18 cm.sup.-3, resulting in continuous oscillation at a wavelength of 671 nm at a room temperature being obtained using a gain-guide type semiconductor laser.
Document 2 describes, in its Abstract, that the fabrication of an AlGaInP type red semiconductor laser using solid materials through the MBE technique has been successfully put into practical use. It is further pointed out therein that reduction of the amount of toxic impurities entering crystal and optimization of a growth temperature are important in order to improve the characteristics of a semiconductor laser device to be fabricated.
The conventional MBE apparatus illustrated in FIG. 7 has three ultra-high vacuum chambers. Specifically, they include two growth chambers 51 and 53, and an intermediate chamber 52 placed between the two growth chambers 51 and 53 so as to connect them to each other. A wafer introducing chamber is further provided so as to be connected with the intermediate chamber 52. A wafer (i.e., a substrate), on which semiconductor lasers are to be fabricated, is introduced into the MBE apparatus via the wafer introducing chamber, and then transferred inside the MBE apparatus from one chamber to the other.
In an actual fabrication process of red semiconductor lasers using the MBE apparatus as shown in FIG. 7, an introduced GaAs substrate is first placed in the growth chamber 51, and a GaAs buffer layer is grown on the GaAs substrate in the chamber 51. Then, the substrate is transferred to the growth chamber 53, and an AlGaInP light-emitting layer is grown in the chamber 53. Thereafter, the substrate is transferred back to the growth chamber 51, and a GaAs cap layer is grown in the chamber 51.
In the growth chamber 53, phosphorus having a high vapor pressure is utilized to grow the AlGaInP light-emitting layer. When a red semiconductor laser is fabricated using such a successive growth manner as described above, phosphorus is required to be effectively evacuated from the growth chamber 53 so as to maintain an ultra-high degree of vacuum in the growth chamber 53.
In order to achieve the above purpose, a turbo-molecular pump 50 (simply indicated as the "turbo pump" in the figure), which is capable of evacuating the growth chamber 53 to an ultra-high degree of vacuum, is utilized for evacuating the growth chamber 53 in addition to a rotary pump. An oil diffusion pump may be used instead of the turbo-molecular pump. In either case, it is important to evacuate phosphorus to the outside of the vacuum chamber 53 at an appropriate rate.
When the above kinds of the pumps are employed, the degree of a vacuum of about 10.sup.-10 mbar can be obtained by simultaneously employing a liquid nitrogen cryopanel (not shown in FIG. 7) disposed along the inner wall of the growth chamber 53.
Ion pumps are respectively employed for the chambers 51, 52 and 53 as well as the wafer introducing chamber in the MBE apparatus in FIG. 7. However, it should be noted that using only absorption pumps such as ion pumps, without employing other types of pumps, is not suitable for evacuating materials having a high vapor pressure such as phosphorus. This is because when the absorption pump is used for a long period of time, an evacuation capacity with respect to the materials (e.g., phosphorus) is saturated due to a high vapor pressure thereof.
During a crystal growth process in the configuration as shown in FIG. 7, a liquid nitrogen cryopanel within the growth chamber 53 and a liquid nitrogen cold trap 54 connected thereto are filled with liquid nitrogen so as to absorb phosphorus molecules, thereby maintaining a high degree of vacuum. However, when the growth process is successively carried out for a long period of time, the amount of adsorbed phosphorus becomes saturated. Thus, it is necessary to pause the growth at appropriate intervals so as to remove the phosphorus-absorbed liquid nitrogen from the liquid nitrogen cryopanel within the growth chamber 53 and the liquid nitrogen cold trap 54, thereby purging the absorbed phosphorus molecules.
FIG. 8 schematically shows a typical fabrication flow of semiconductor lasers in a conventional technique using the conventional MBE apparatus as described above.
In this fabrication flow, a wafer growth process, in which semiconductor laser devices are completed, requires about 3.6 hours. Typically, such a wafer growth process is repeated for 20 cycles, as indicated by 20 elliptic marks in the figure. The total time required for the 20 cycles of the wafer growth process is therefore about 72 hours (=3.6 hours.times.20), i.e., about 3 days.
After the 20 cycles of the wafer growth process are completed, the fabrication of new semiconductor lasers is ceased, and the step of purging the phosphorus adsorbed in the liquid nitrogen cryopanel is conducted for about 8 hours. Subsequently, the step of purging the phosphorus adsorbed in the liquid nitrogen cold trap 54 is conducted for about 20 hours.
As shown in FIG. 8, it takes about 1 day or longer to complete the step of purging the absorbed phosphorus after the crystal growth process of about three successive days. In other words, it is necessary to cease the fabrication of red semiconductor lasers for almost 1 day in order to complete the purging steps. This prevents improvement of a production efficiency of the red semiconductor lasers.
If the crystal growth process is successively performed over a longer period of time than described above, an evacuation efficiency of phosphorus from the chambers becomes saturated. In addition, it requires a longer period of time for the absorbed phosphorus to be purged. Therefore, a production efficiency is further lowered.